It is often desirable in manufacturing processes to apply a plurality of printed, or otherwise patterned, layers to a surface of an object using various application techniques such as pad-printing, lithography and the like. For example, in printing processes a page may contain multiple layers of ink to generate a multi-colored image. Many other objects, such as printed-iris contact lenses, semiconductor wafers, and even golf balls may also contain a multi-layer printed pattern on at least a part of their surface. These patterns may or may not overlap, in at least some locations of the surface. In such processes, manufacturers attempt to minimize defects, which can occur (for example) due to misalignment among the layers or due to extra or missing ink in one or more of the layers.
One approach to minimizing defects is to perform visual inspection of a printed surface using a machine vision system, (also termed simply a “vision system” herein). In a single-layer structure this can be accomplished using a so-called “golden template” comparison (GTC), in which a reference image (the golden template) is generated from one or more images of good parts. The golden template image is subsequently subtracted from the image of each part to be inspected to produce an intensity or color “difference image”. Global or regional amplitude differences between the golden template and the runtime image may be normalized away prior to subtraction. And the difference image can either be signed or absolute. Significant differences are usually associated with defects in the runtime part. Regions of difference can be detected and quantified by a variety of techniques, such as blob segmentation and connectivity analysis. If differences exceed acceptable intensity or size thresholds, then the part can be considered defective.
However, the application of standard GTC processes to a multi-layered print surface is significantly more problematic. For example, overlap between layers that vary from object to object (because of differing alignment among layers) are not easily resolved by a single golden template. As such it is desirable to provide a system and method for generating a golden template that allows a multi-layer printed/patterned surface (typically having a differing color in each layer) to be inspected by a vision system.